The present invention relates to an improvement in a glowbottle starter for lamps. Glowbottle starters are well-known gaseous discharge forming devices which have been used commercially to ignite lamps for at least fifty years. Such a device consists of a small hermetically-sealed vial containing two spaced electrodes, at least one of which is a bimetal. The air has been exhausted from the interior of the vial and replaced by a selected gas at a predetermined pressure. In use, one electrode terminal of the glowbottle is connected to one end of a first filament of the fluorescent lamp and the other electrode terminal is connected to one end of the other (second) filament of the fluorescent lamp. The high voltage output of the power circuit energizing the lamp, a ballast, is connected to the other end of the first filament. The current return lead of the ballast is connected to the other end of the second filament.
When a high potential is applied by the ballast to the combination of the lamp, the glowbottle and the starter, the two filaments of the lamp and the interelectrode gap of the glowbottle are effectively in series connection across the high potential. The electrode surface composition, the interelectrode gap and gas filling pressure of the glowbottle are selected so that the gas within the glowbottle ionizes and a cold-cathode glow discharge is established between the two electrodes. The ion bombardment of the glow discharge heats the bimetal electrode causing it to deflect and contact the other electrode of the glowbottle to establish a short-circuit connection as with the closure of a switch.
The lamp-glowbottle circuit then has the two lamp filaments connected through the closed glowbottle switch in series across the output terminals of the ballast. A current flows through them, limited primarily by the impedance of the ballast. The filaments are heated to thermionic emission temperatures by the flowing current, but the bimetal in the glowbottle is substantially not heated since its resistance is deliberately made low to minimize such heating. Thus, the bimetal cools, "unbends", and ceases to contact the other electrode of the glowbottle, thereby opening the switch. Since the ballast impedance includes an inductive component, the interruption of current by the opening of the switch generates a high transient pulse voltage. The high voltage pulse appears across the fluorescent lamp interelectrode gap, with the lamp electrodes heated to thermionic-emitting temperatures. The lamp ignites and current flows through the fluorescent lamp, determined by the ballast open circuit voltage, the lamp operating voltage at the operating current, and the ballast series impedance. The potential now appearing across the glowbottle terminals is the lamp operating voltage, typically less than one half the ballast open-circuit voltage. The gap and pressure parameters of the glowbottle are chosen in such a way that this potential is inadequate to cause ionization and breakdown therein. Thus the glowbottle remains inert and passive until the fluorescent lamp is extinguished and is re-started during its next operating cycle.
The design and manufacturing requirements on the glowbottle are: (1) it must ionize and conduct and close its bimetal switch at the open-circuit voltage generated by the ballast power circuit at the lowest value of line voltage permitted by the specified tolerances and (2) it must not ionize, conduct, or close its bimetal switch at the highest value of lamp voltage permitted by the specified tolerances. These two requirements establish stringent conditions on interelectrode gap, fill gas composition and pressure, and electrode surface composition in the glowbottle.
Despite the relative complexity of the lamp ignition process and the demanding conditions imposed on glowbottle characteristics, this lamp-ballast-starter system has been extensively refined and improved in performance, reliability and cost over the last half century to the extent that it is the least expensive system for igniting and controlling fluorescent lamps and is the preferred system wherever 240 volts is the predominant local distribution voltage and the ballast is a simple series choke. It is, in addition, the preferred system for operation of so-called compact fluorescent lamps even on 120-volt power distribution systems. It is commonplace to include the glowbottle starter within the base of a compact fluorescent lamp, so that only a series inductive choke is required in the fixture to connect the lamp to the power grid.
The above list of advantages notwithstanding, the glowbottle starter device suffers from an intrinsic problem which may lead to undesirable constraints in design parameters and which may compromise performance of the glowbottle. This stems from the fact that the breakdown and ionization process within the glowbottle itself depends on the presence of at least one free electron in the gas to initiate a Townsend avalanche and trigger the breakdown and subsequent current flow that causes bimetal closure. If there is no free electron available at the time that open-circuit voltage is applied, nothing happens. The ionization and glow current through the glow bottle is delayed until at least one free electron appears. Thus there is a delay between the application of voltage and the commencement of the starting cycle. The maximum time delay between application of open-circuit voltage and glowbottle closure is a performance specification for many systems. For other systems, the time delay between application of open circuit voltage and fluorescent lamp ignition is specified. However, even for this specification, the delay time between voltage application and glowbottle closure may be an important fraction of the allowed starting time.
A problem which arises in this connection is the so-called photoelectric effect. The presence of ambient light incident on the glowbottle can result in the emission internally of photoelectrons from electrode or glass surfaces. These photoelectrons may then provide the free electrons needed for initiation of Townsend avalanches and the breakdown and ionization within the glowbottle. In the absence of light, there is no such contribution to the provision of initial electrons. Therefore, the time delay in ionization may depend on whether the glowbottle is in the dark or in an illuminated location.
Furthermore, the breakdown and ionization process is statistical in nature. If there are many thousands of initial electrons, each of which produces an electron avalanche and only one such avalanche needs to successfully liberate new electrons from the cathode thereby regenerating itself, the mean breakdown voltage can be lower than if there is only one free electron. For successful breakdown with one initiating electron, the probability must be 100% that the avalanche creates enough free electrons and enough ions returning to the cathode to liberate one additional free electron thereby generating a new avalanche. To achieve this will require higher electric field strength and higher voltage across the gap than would be required if there were a million free electrons and the probability of a "successful" avalanche needed only to be one in a million. Therefore, in addition to the time delay intrinsic in having to wait for a free electron under conditions of a limited supply, there is also a higher mean voltage required to insure reliable breakdown. In the domain of operation of glowbottles, breakdown voltage increases with increasing product of pressure times gap separation, that is the "pd-product". When there is a very limited supply of initial electrons, the pd-product of the bimetal interelectrode gap must be lower to insure reliable breakdown at a given voltage than would be the case if there were an ample supply.
In the practical sense, this means that in the absence of some method to generate free electrons, the gap pd-product must be reduced to provide near 100% breakdown probability at the required "closure voltage" with the glowbottle in the dark. However, once the fluorescent lamp lights, the glowbottle is usually somewhat illuminated by light from the lamp itself tending to lower its average breakdown voltage. It may happen that with the bottle illuminated, the breakdown voltage resulting from the combination of ample supply of initial electrons plus the reduced pd-product may be lowered to the extent that some bottles will ionize and reclose at the operating voltage of the lamp. This is especially a problem with compact fluorescent lamps, which have a much higher reignition peak voltage every half cycle relative to rms operating voltage than do conventional large diameter lamps. The combination of high peak voltage and lowered breakdown voltage due to photoelectrically-emitted initiating electrons may lead to ionization and possible reclosure during operation. This possible tendency to reclose is of course increased if the pd-product had to be chosen to reduce average breakdown voltage to insure reliable breakdown with only one initiating electron. In any case, the lowering of mean breakdown voltage with the lamp lit will reduce the difference between glowbottle breakdown voltage and operating voltage of the lamp requiring tighter manufacturing tolerances to insure reliable closure with the glowbottle in the dark and non-closure with the glowbottle illuminated.
For these reasons it is common in glowbottle manufacture to seek to provide an ample supply of free electrons to initiate breakdown with the glowbottle in the dark. Thus, the presence of additional free electrons from photoelectric emission when the glowbottle is illuminated will have negligible effect on breakdown voltage. There will be substantially no difference in mean breakdown voltage between bottle-illuminated and bottle-in-the-dark states. Thereby, there will be no narrowing of tolerances to insure reliable closure in the dark simultaneously with reliable non-reclosure with the glowbottle illuminated. Moreover, the provision of an ample supply of free electrons to initiate the breakdown in the glowbottle will eliminate the so-called "statistical time lag" between the application of voltage and the closure of the bimetal switch. This will enhance the ease of meeting the starting time specifications for the bottle-in-the-dark state.
It is known to the prior art to employ radioactive substances within the glowbottle to provide initial electrons. Commonly the radioactive isotope krypton-85 is added to the fill gas in nanocurie to microcurie amounts. Beta-rays of 0.72 MEV energy from disintegrating Kr-85 nuclei produce copious ionization within the gas, which furnishes a substantial supply of free electrons to initiate Townsend avalanches and cause breakdown. An isotope of hydrogen, tritium, has also been employed as a gaseous dopant. Its beta-rays of 18 KEV energy also produce ionization in the gas. An advantage of tritium over krypton-85 is that no gamma-rays are emitted by tritium. Therefore, all the ionizing radiation emitted is contained within the glowbottle, since the beta-rays are not transmitted through the glass. In addition to beta-rays, krypton-85 emits 0.54 MEV gamma-rays, which penetrate through the glass wall of the glowbottle, the metal or plastic cover of the glowbottle, and any other housing which may be present. Thus, ionizing radiation is deposited in the space surrounding the glowbottle offering the possibility of radiation hazard. It is common in the prior art to point out that because of the very low level of radioactivity required, the ionizing radiation delivered outside the glowbottle is negligible in comparison to normal background radiation, and there is therefore no radiation hazard. Regulatory authorities have agreed as to a lack of hazard and do not require "Radioactive" labels to be affixed to products or packages.
It is also known to the prior art to dispose thorium in some form within the glowbottle, for example, thorium oxide, thorium metal or the like. Thorium has several long-lived radioactive isotopes, emitting alpha, beta, and gamma rays, forming daughter products which are themselves radioactive. The alpha and beta rays produce copious ionization within the gas, providing free electrons to serve as initial electrons in the breakdown process. Again, the gamma component of the radiation escapes the glowbottle to the ambient, but again the quantity of radiation is so small that the prior art has considered there to be negligible radiation hazard.
Although there is insignificant radioactive hazard from glowbottles in service in the marketplace, there is a radioactive hazard in manufacture. Relatively large quantities of krypton-85, tritium or thorium must be handled safely in the manufacture of hundreds of millions of glowbottles every year. Bulk shipment or warehousing of glowbottles in quantity may result in the presence at a single location of significant quantities of radioactive material.
It is apparent that some other means of providing initiating electrons without the use of radioactive materials would be highly desirable, but has not been available to manufacturers.